Studies on Improving the Polarographic Method for Determining Trace Impurities of Zinc, Lead, and Copper in High-Purity Indium with Separation of Excess Indium by Extraction with Diethyl Ether

N. A. Mesyats, L. F. Zaichko, R. S. Tyutyunkova, M. S. Zakharov

(Presented by the scientific seminar of the Department of Physical and Colloid Chemistry)

This work was a continuation of the method previously developed by us for determining lead and copper in high-purity indium [1]. An alkaline ethylenediamine solution served as the supporting electrolyte for determining the indicated impurities. As later studies showed, this supporting electrolyte is unsuitable for routine analyses, since ethylenediamine requires prolonged purification by electrolysis, during which it partially decomposes and creates difficulties for polarography. We therefore carried out studies to improve this method. The extractant used was the same (diethyl ether). A tartrate–ammonia supporting electrolyte was chosen (0.1 M tartaric acid + 0.25 M ammonium acetate). The reagents for preparing this supporting electrolyte are of chemically pure grade and do not require additional purification; they are stable during the work. The peak potentials of the elements studied are: Zn — −0.96 V (sat. cal. el.); In — −0.6 V (sat. cal. el.); Pb — −0.45 V (sat. cal. el.); Cu — −0.2 V (sat. cal. el.). In order that indium would not interfere with the determination of lead and copper, after its peak was reached (−0.6 V/sat. cal. el.) the potentiometric drum was stopped for complete dissolution of the indium, and then the peaks of lead and copper were recorded.

Earlier [1] we established the extraction time necessary for quantitative separation of the Pb and Cu impurities from indium. This time was 9 minutes, i.e., a triple extraction was carried out, 3 minutes each. With such an extraction time, part of the impurities (especially zinc) is adsorbed on the surface of the funnel during shaking, and losses during determination amount to ~50% (for Zn). In this connection, we carried out studies to determine the optimum extraction time. It was chosen on the basis of two conditions: first, that Zn losses be minimal, and second, that less indium remain, since it interferes with the subsequent determination of impurities (especially Zn) as a result of the large residual current. We studied the effect of an excess amount of indium on the shape and magnitude of the Zn peak. The indium concentration was varied in the range from 5 μg/ml to 700 μg/ml. It was found that indium at concentrations up to 300–400 μg/ml does not exert a significant influence on the Zn peak; a higher indium concentration, on the order of 600–700 μg/ml, screens the Zn peak. That is, during extraction, 99.92% of the total amount of indium must be removed from the solution, and only 0.08% must remain so as not to interfere with the determination of the trace impurities Pb, Cu, and especially Zn.

The optimum extraction time was selected: triple extraction for 1.5 min, 2 min, and 2 min. Under these conditions, when extraction is carried out with once-distilled diethyl ether, a very large amount of indium remains, which makes determination of the impurities almost impossible (\(\sim 1800\text{–}1900\ \mu\text{g}/\text{ml}\)). For partial removal of the excess amount of indium, we tried igniting the residue after evaporation of the acid phase at the volatilization temperature of indium bromide \(\mathrm{InBr_3}\) (\(\sim 371^\circ\text{C}\)). The volatilization temperatures of the bromides of the other metals are much higher: \(\mathrm{ZnBr_2}\)—702°, \(\mathrm{PbBr_2}\)—918°, \(\mathrm{CuBr_2}\)—1357°, \(\mathrm{InBr_2}\)—630°. In order that most of the indium pass into \(\mathrm{InBr_3}\), when the indium samples were dissolved in 5 M HBr, 0.3–0.5 ml of \(\mathrm{Br_2}\) was added. By igniting the residue for 10–15 minutes at \(t^\circ = 370^\circ\text{C}\text{–}375^\circ\), an insignificant amount of indium remained (\(\sim 400\ \mu\text{g}/\text{ml}\)), and determination of Zn, Pb, and Cu was possible. But in determining the volatility of Zn at high temperature it was found that at a temperature somewhat above \(\sim 380\text{–}390^\circ\), part of the Zn may volatilize together with the indium; therefore, this method of removing indium requires very careful maintenance of a definite temperature and is not entirely convenient for systematic analyses. Consequently, although this route was considered positive, it did not yield a reliable and convenient procedure for determining impurities in high-purity indium.

After carrying out these studies, we tried another route for partial removal of the excess indium. For this purpose a new supporting electrolyte was taken (0.03 M \(\mathrm{H_3PO_4} + 0.25\) M ammonium acetate), i.e., phosphoric acid was taken instead of tartaric acid, whose phosphate ion suppresses the indium wave (\(\sim 20\)-fold). The effect of the concentration of phosphoric acid on the magnitude of the Zn wave was investigated, and it was found that at a concentration of 0.03 M the phosphate affects neither the Zn wave nor the Cu and Pb waves; only indium is suppressed. On the polarogram, an indium wave comparable with the other waves is obtained. But when working with phosphoric acid, various difficulties arise, associated with the formation of films on the surface of the mercury and the appearance of an unknown wave in the potential region lying between the lead and copper peaks (\(-0.25\) V vs. SCE). As study of this wave showed, it is formed not by some metal in solution, but precisely by a phosphate film, and it makes determination of copper difficult. Therefore, this supporting electrolyte can be used successfully only for determining Zn and Pb.

Table 1

Results of determination of microimpurities of Zn and Cu in high-purity indium.
Melt No. 27

We tried one more route: improving the quality of the diethyl ether. Despite literature references [2] that diethyl ether is unsuitable for the quantitative extraction of indium, we carried out additional treatment of diethyl ether in order to improve the completeness of indium extraction. For this, once-distilled ether was treated with metallic sodium or ignited calcium chloride, followed by distillation in a molybdenum apparatus. When this ether was used, satisfactory separation of the bulk of the indium was obtained. The remaining amount is 200–190 \(\mu\text{g}/\text{ml}\), i.e., 0.4% of the total amount of indium. In the presence of such an amount of indium it is possible

polarography in a tartrate–ammonia supporting electrolyte (\(\mathrm{pH} = 6\)). Using the proposed procedure, a series of analyses of indium samples was carried out to determine their Zn and Cu content (Table 1).

REFERENCES

1. M. S. Zakharov, N. A. Mesyats. Determination of microconcentrations of copper, lead, and bismuth in indium with separation of excess indium by extraction. In: Methods for the Analysis of Chemical Reagents and Preparations, issue 5–6. IREA Publishing House, 1963.

2. S. I. Sinyakova, A. R. Dudareva et al. Determination of impurities of copper, lead, cadmium, and zinc in especially pure indium and its salts by the method of amalgam polarography with a stationary electrode. Vol. XVIII, issue 3, Publishing House of the Academy of Sciences of the USSR, 1963.